v0.1.5

Introduce the Formatter interface. Example.

• Add Logger.RegisterFormatter(Formatter) to register a custom Formatter.
• Add Logger.SetFormat(formatter string, opts ...interface{}) to set the default formatter for all log levels.
• Add Logger.SetLevelFormat(levelName string, formatter string, opts ...interface{}) to change the output format for the given "levelName".
• Remove the golog.JSON handler, introduced 12 hours ago, as it's now useless.